1. Field of the Invention
The present invention relates generally to a strategy to control battery state of charge (SOC) and specifically to control the battery SOC based on vehicle velocity.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV""s potential benefits,
An HEV can use batteries to store electrical energy for use by the vehicle""s electric motor. Active control of the HEV battery becomes a critical vehicle function to achieve the HEV goals of reduced emissions and fuel economy. Such active battery control cannot only increase overall vehicle performance and fuel economy but also increase battery life.
Simple passive battery controllers are known in the prior art that respond only to battery voltage. The simplest controllers fail to maintain the batteries at a controlled charge level. The batteries slowly discharge during operation. Further, a passive battery controller is inefficient because it cannot control the battery operating point.
Battery state of charge (SOC) controls for vehicles are known in the prior art using various conditions or criteria. See generally, U.S. Pat. No. 5,969,624 to Sakai et al. (battery cell voltage); U.S. Pat. No. 5,945,808 to Kikuchi et al. (battery temperature and the current SOC); U.S. Pat. No. 6,091,228 to Chady et al. (uses at least integration and produces a signal source representing the desired auxiliary source current); U.S. Pat. No. 4,187,486 to Etienne (excitation winding of the generator in accordance with the SOC); U.S. Pat. No. 5,264,764 to Kuang (power signal generated when the battery SOC is less than a predetermined intermediate value and the energy consumed by the electric drive system is greater than the electric energy delivered by the range extender); U.S. Pat. No. 5,115,183 to Kyoukane et al. (circuit to control engine speed and predetermined generator speed); U.S. Pat. No. 5,550,445 to Nii (detects various conditions for heavy loads); and U.S. Pat. No. 4,313,080 to Park (uses battery voltage).
Battery controllers using vehicle speed are also known in the prior art. U.S. Pat. No. 4,682,097 to Matsui describes battery charging as a function of the difference between vehicle speed and the charge top speed. In the invention, a xe2x80x9cbalancexe2x80x9d vehicle speed, the speed at which mean charging current and mean discharging current are balanced, is obtained. Next a xe2x80x9ccharge stopxe2x80x9d vehicle speed corresponding to the balance vehicle speed is calculated as a reference value. When the charge stop speed is greater than the actual vehicle speed, battery charging is stopped.
Many improvements and variations to prior art battery controls are possible. For example, Matsui only calibrates the charge stop speed as a linear function of the vehicle velocity. New ways to control battery SOC including both harging and discharging when appropriate need to be developed using an active control strategy. This type of application would be particularly useful in a hybrid electric vehicle.
Accordingly, an object of the present invention is to provide an improved strategy for actively controlling battery state of charge (SOC) as a function of the vehicle""s kinetic energy.
The present invention provides a strategy for a vehicle system controller (VSC) to vary a target battery SOC as a function of vehicle speed based on either vehicle kinetic energy or the square of vehicle speed. The VSC can set a maximum battery SOC limit using vehicle velocity and actively control a rate of charging the battery and determine a target battery SOC as a balance between the charge time and the charge current.
As part of the strategy to actively control the battery SOC, the VSC can determine a target battery SOC based on a predetermined constant and a predetermined offset value. The strategy can increase target battery SOC as vehicle speed increases until the maximum battery SOC limit is reached.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.